FIG. 2 is a cross-sectional drawing of a conventional thin-film EL element. The thin film EL element has a double insulating structure, wherein a transparent electrode 12, first insulating layer 13, EL light-emitting layer 14, second insulating layer 15, and back electrode 16 are all prepared on a glass substrate 11. The light-emitting layer 14 is made of II-VI group element sulfide, to which doping material yielding luminescent centers such as a small quantity of Mn, Tb, Sm, Tm, Pr, etc. are added thereto. The II-VI group element sulfide utilized may include zinc sulfide (ZnS), calcium sulfide (CaS), strontium sulfide (SrS), and others.
Several different methods have been examined for preparing the light-emitting layer, including the vacuum evaporation method, the atomic layer crystal growth evaporation (ALE) method, which is one of the CVD methods, and the sputtering method. Since the sputtering method can prepare a uniform film having a large area, it is most suitable for mass production.
Light-emitting layers prepared by the sputtering method, however, are inferior in light-emitting luminance (See ACTA POLYTECNICA SCANDINAVICA, Applied Physics Series, No.170 "5th International Work Shop on Electrochemistry" pp. 41-48). Further examination of the sputtering method by applicants has shown that zinc, manganese, and sulfur are different from one another in physical properties such as sputter rate and vapor pressure: that actual film composition is considerably different from target composition; and that the crystalline nature of the light-emitting layer is degraded as a result.
Reactive sputtering is expected to be a method for obtaining a uniform and high quality light-emitting layer. Japanese laid-open application No. 62-271396 discloses a reactive sputtering method using a zinc target and hydrogen sulfide gas. A uniform and high quality light-emitting layer is prepared by using an inactive gas containing 5-20% hydrogen sulfide (H.sub.2 S) by volume and by setting substrate temperature to 100.degree.-350.degree. C. A particularly good result is obtained when the substrate temperature is set to a range of 200.degree.-250.degree. C., the hydrogen sulfide concentration is set to 10%, the electric discharge pressure is set to 10 mTorr, and the electric discharge power is set to 8 W/in.sup.2 (1.2 W/cm.sup.2).
A light-emitting layer produced under the above conditions, however, has a film-forming rate of about 13 nm/min, which is too slow for practical use. While a high film-forming rate can be achieved by using a high electric discharge power, a light-emitting layer having good characteristics cannot be obtained simply by increasing electric discharge power. Increasing electric discharge alone increases only the quantity of zinc and manganese supplied; the supply of sulfur does not keep balance with the supply of zinc or manganese. As a consequence, the composite ratio of zinc to sulfur in the film is no longer 1 to 1, and a light-emitting layer having good characteristics cannot be obtained.